Make a variable called timing with default 0.

#alias {timing} {#if @timing=0 {timing=1} {timing=0}}

Trigger your prompt to:

#if (@timing=1) {#sayp {--/%time( hh:mm:ss.zzz)"/ "}}

Now if you hit timing alias, it'll show timestamps on your prompt, if you hit it again, it won't.

Enjoy!